1. Field of the Invention
This invention is directed to electrical circuit breakers, and more particularly to electrical circuit breakers which incorporate a blocking mechanism for restricting travel of a cradle assembly of the circuit breaker during ON to OFF operation without affecting normal reset or trip operation of the circuit breaker.
2. Background of Information
Molded case circuit breakers are generally old and well known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,489,295; 4,638,277; 4,656,444; and 4,679,018. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and relatively high level short circuit. An overload condition is about 200-300% of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000% or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers include a pair of separable contacts per phase which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation the contacts may be opened by an operating mechanism or by a magnetic repulsion member. The magnetic repulsion member causes the contacts to separate under relatively high level short circuit conditions. More particularly, the magnetic repulsion member is connected between a pivotally mounted contact arm and a stationary conductor. The magnetic repulsion member is a generally V-shaped member defining two legs. During high level short circuit conditions, magnetic repulsion forces are generated between the legs of the magnetic repulsion member as a result of the current flowing therethrough which, in turn, causes the pivotally mounted contact arm to open.
In a multi-pole circuit breaker, such as a three-pole circuit breaker, three separate contact assemblies, one for each pole, having magnetic repulsion members are provided. The contact arm assemblies are operated independently by the magnetic repulsion members. For example, for a high level short circuit on the A phase, only the A phase contacts would be blown open by its respective magnetic repulsion member. The magnetic repulsion members for the B and C phases would be unaffected by the operation of the A phase contact assembly. The circuit breaker operating mechanism is used to trip the other two poles in such a situation. This is done to prevent a condition known as single phasing, which can occur for circuit breakers connected to rotational loads, such as motors. In such a situation, unless all phases are tripped, the motor may act as a generator and feed the fault.
In the other automatic mode of operation, the contact assemblies for all three poles are tripped together by a current sensing circuit and a mechanical operating mechanism. More particularly, current transformers are provided within the circuit breaker housing to sense overcurrent conditions. When an overcurrent condition is sensed, the current transformers provide a signal to electronic circuitry which actuates the operating mechanism to cause the contacts to be separated.
The operating mechanism of the circuit breaker is designed to rapidly open and close the separable contacts, thereby preventing a moveable contact from stopping at any position which is intermediate a fully open or fully closed position.
The circuit breaker includes a pivoting operating handle, which projects through an opening formed in the breaker housing, for manual operation. The handle may assume one of four positions during normal operation of the circuit breaker. In an ON position, the handle is positioned at one end of its permissible travel. When the operating handle is moved to this position, and the breaker is not tripped, the contacts of the circuit breaker close, thereby allowing electrical current to flow from a current source to an associated electrical circuit. Near the opposite end of travel of the handle is an OFF position. When the handle is moved to that position, the contacts of the circuit breaker open, thereby preventing current from flowing through the circuit breaker.
A third position is a trip position which is approximately midway between the ON position and the OFF position. The handle automatically assumes this position whenever the operating mechanism has tripped the circuit breaker and opened the contacts. Once the circuit breaker has been tripped, the electrical contacts cannot be reclosed until the operating handle is first moved to a fourth reset position and then back to the ON position.
The reset position, which is beyond the OFF position, is at the opposite end of travel of the handle with respect to the ON position. When the handle is moved to the reset position, a trip mechanism is reset in preparation for reclosure of the contacts when the handle is moved back to the ON position.
A typical example of a circuit breaker is disclosed in U.S. Pat. Nos. 5,200,724 issued Apr. 6, 1993 to Lance Gula et al. entitled "Electrical Circuit Breaker Operating Handle Block" and assigned to the assignee of the present invention, which is herein incorporated by reference.
Whenever the handle is in the ON position, biasing springs connected to the handle provide a biasing force to a pivot point. The pivot point pivotally connects upper and lower links of a toggle mechanism. The lower toggle link is also pivotally connected to an arm carrier carrying the movable main contacts. The upper toggle link is pivotally connected to a cradle which can be latched by a cradle latch mechanism which cooperates with the trip mechanism. When the circuit breaker is tripped, and the cradle is unlatched, the cradle rotates under the influence of the biasing springs. With the rotation of the cradle, the biasing springs also cause the collapse of the toggle mechanism. In turn, this causes the separation of the main contacts.
After a trip, whenever the handle is rotated toward the reset position, an internal reset pin interconnected with the handle rotates toward a surface of the cradle. As the reset pin engages this surface, the cradle, which is in an unlatched position, is rotated toward a latched position. As the cradle rotates, it engages a lever of a reset mechanism. The reset mechanism rotates a trip bar to engage the cradle latch mechanism. In turn, the latch mechanism latches the cradle in its latched position. After this reset operation, the circuit breaker handle may be moved to the ON position, thereby closing the contacts.
Under manual operation of the circuit breaker, whenever the handle is moved from the ON to the OFF position, the combination of the handle in conjunction with the attached biasing springs operate to separate the movable main contacts from the fixed main contacts of the circuit breaker. As the handle is moved, the biasing force of the biasing springs causes the collapse of the toggle mechanism. The biasing force also causes the upper toggle link to pivot in one direction about the pivot point. In a related manner, the lower toggle link pivots in the opposite direction about the pivot point. The lower toggle link, in turn, raises the arm carrier which separates the main contacts.
When the toggle mechanism collapses, as the handle is manually moved toward the OFF position, the stored energy in the spring provides additional force, beyond the force of manual operation, to further rotate the handle toward the OFF position. As the handle rotates, the handle reset pin also rotates toward the cradle surface in a manner similar to the reset operation described above. As the reset pin engages the cradle surface, the cradle, which is in its latched position, is rotated in a comparable manner as the reset operation. As the cradle is rotated, beyond its latched position, it reaches its reset position. In a manner similar to the reset operation, the cradle engages the lever of the reset mechanism and rotates the trip bar to engage the latch mechanism. At this point, the reset mechanism cannot adversely affect normal operation of the circuit breaker. This is because, during the reset operation preceding the placement of the handle in the ON position, the latch mechanism had engaged the cradle in its latched position.
However, some conditions, that need improvement, may result whenever the handle is moved from the ON to the OFF position. In particular, whenever the trip bar is rotated by the lever of the reset mechanism, an over-travel spring of the trip bar is compressed. Furthermore, tripping mechanisms connected to the trip bar, such as a flux transfer shunt trip and an attachment tab, are also compressed. In turn, the compression of the over-travel spring and the tripping mechanisms causes a counter-rotation of the trip bar. Under certain circumstances, such counter-rotation causes the trip bar to reach a trip position. Under such circumstances, the latch mechanism unlatches the cradle and, thus, the circuit breaker enters the trip position. This necessitates a reset of the circuit breaker before the circuit breaker can be placed in the intended OFF position.
There is a need, therefore, for a mechanism that prevents the cradle from entering the trip position during manual operation of the circuit breaker handle from the ON to the OFF position.
There is a more particular need for such a mechanism that does not affect normal reset operation of the circuit breaker.
There is also a more particular need for such a mechanism that does not affect the operation or loading of the circuit breaker tripping mechanisms.
There is yet another more particular need for such a mechanism that does not alter the handle forces required for ON to OFF, or trip to reset, operation of the circuit breaker.
There is still another more particular need for such a mechanism that withstands handle forces during OFF to reset operation, after ON to OFF operation, of the circuit breaker.